OCR Government and PoliticsWatch
The thing is with Government and Politics is in terms of knowledge, things aren't that hard to understand. So is it really necessary to have a revision guide?. For example, you'll do a topic on electoral systems. Google a type of electoral system, such as First Past the Post and there will be tons on it. For example, this. http://aceproject.org/ace-en/topics/...sd01a/esd01a01 - Its no different to what a revision guide will give you.
Another thing with the Government and Politics course is that in order to write a really good essay you need to include relevant examples. They're encouraged to be examples that have been relevant in the news and new as well. No revision guide will give you that, only old examples that examiners will read tediously one after another.
If you insist on a textbook, I'd recommend this one. Its the one we use and has pretty much everything covered. Relying on this for your knowledge and examples wouldn't be advisable though, but no revision guide/textbook will be.